Design of a Megawatt-Scale IGCT-Based Medium-Voltage Direct Current Transformer

This paper presents a robust design of a megawatt-scale medium-voltage (MV) direct current transformer (DCT) that features a bidirectional LLC resonant converter topology. This topology harnesses the inherent ability to control power flow seamlessly between two MVDC buses without relying on closed-loop control, maximizing efficiency and stability. Integrated gate-commutated thyristors (IGCTs) were chosen as the power devices due to their superb on-state properties, enhancing the performance of this resonant DCT. The power stages incorporate a three-level neutral point clamped (3L-NPC) topology, paired with a split-capacitor DC link, enabling three-level modulation during soft-start and two-level modulation during normal operation. Taking advantage of the resonant operation, the record-high switching frequency in the range of 5kHz is achieved. The custom-designed medium frequency transformer (MFT) uses oil-insulated copper pipes for the windings and nanocrystalline core while integrating the inductive elements of the resonant tank. Power-in-the-loop experiments are presented that demonstrate the system efficiency approaching 99%.